[SelectionDAG][VP] Fix MemSDNode::getBasePtr

Found while working on D108987. When interpreting VP nodes as
`MemSDNode` nodes, this function would return the incorrect indices.
This was due to `VP_GATHER` and having no "passthru", and both
`VP_GATHER` and `VP_SCATTER` having their mask operands *after* the base
pointer, unlike `MGATHER` and `MSCATTER`.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D109308